1. Field of Invention
This invention relates to a PWM signal generator and, more particularly, to the PWM signal generator which is able to control a rotating speed of a motor to drive an optical disk.
2. Description of Related Art
A rotating speed of a motor is controlled in an optical disk apparatus such as a CD player or a DVD player. The rotating speed is controlled by using a PWM (Pulse Width Modulation) signal.
An analog PWM signal generator and a digital PWM signal generator are known as a PWM signal generator. The analog PWM signal generator generates an analog PWM signal by comparing an analog modulating wave (signal wave) with a triangular wave (career wave). The digital PWM signal generator generates a digital PWM data signal using a triangular career wave which is generated based on a count value counted by an up-down counter. A general motor control circuit which has the analog PWM signal generator is described below.
FIG. 7 shows the motor control circuit 40 having the analog PWM signal generator. The motor control circuit 40 is described in Japanese Unexamined Patent Application Publication No. 05-49263. (Tagami) As shown in FIG. 7, the motor control circuit 40 has a plurality of PWM signal generators 41 and a plurality of motor drive signal generators (motor driver) 42. Each PWM signal generator 41 corresponds to a different modulating wave (sine waves A, B and C). The sine waves A, B and C have different phases respectively. The PWM signal generator 41 receives the sine modulating wave and a triangular career wave T, and generates a pair of PWM signals. As shown in FIG. 7, there are three PWM signal generators. Therefore three pairs of PWM signals (UP and UN, VP and VN, WP and WN) are generated. The motor driver 42 generates drive signals (BUP, BUN, BVP, BVN, BWP and BWN). These drive signals correspond to the PWM signals respectively.
The drive signals (BUP,BUN,BVP,BVN,BWP and BWN) generated by the motor control circuit 40 are input to switching transistors 51 of a motor drive circuit 50 which is shown in FIG. 8. The voltage of each line U, V and W is varied depending on the conductive states of the switching transistors 51. Thereby, the rotating speed of the motor 52 is controlled.
FIG. 9 shows the drive signals which are generated by the motor control circuit 40 and voltage wave forms between lines. The voltage wave forms shown in FIG. 9 are applied between lines U and V, V and W, W and U. Referring to FIG. 9, the PWM signal generator 41 compares the triangular career wave T with sine modulating waves A, B and C. The PWM signal generator 41 generates PWM signals UP, VP and WP which are H level while an instantaneous value of each sine wave is higher than an instantaneous value of the career wave. The PWM signal generator 41 also generates inversed PWM signals UN, VN and WN which are inversed signals of the PWM signals UP, VP and WP. Each drive signal BUP, BUN, BVP, BVN, BWP and BWN which are output from the motor control circuit 40 corresponds to the each PWM signal generated by the PWM signal generator 41.
The drive signals BUP, BUN, BVP, BVN, BWP and BWN are input to the motor drive circuit 50. The voltages between lines shown in FIG. 9 are applied to the motor 52. Therefore, an average voltage between lines which corresponds to the modulating wave is applied to the motor 52, and the motor rotates with a certain rotating speed which is directed by a frequency of the modulating wave.
The use of the digital PWM signal generator is getting popular instead of the use of the analog PWM signal generator in generating the PWM signals UP, VP and WP. A DSP (Digital Signal Processor) is used in generating a digital PWM data (pulse width data). The DSP generally controls not only PWM generators but also other portions of an apparatus. (The DSP is shared as a control portion of other portions of an apparatus)
An operating speed of the DSP is getting faster recently, and therefore a frequency of the career wave which is generated by the up-down counter inside of the DSP is getting higher. As a resolution of the PWM signal is improved, the minimum pulse width of the PWM signal is getting narrower.
However, in accordance with the narrowing of the minimum pulse width, there are some cases that the motor drive circuit is not able to respond to the narrow pulse width even in the maximum response speed of the motor drive circuit.
In these cases, pulses having a narrow width may be ignored by the motor drive circuit. If the pulses are ignored, the drive signal is not generated and the motor does not operate properly. Even if the pulses are not ignored, pulses are damped in the motor drive circuit and the PWM signals are distorted. Therefore, a linear drive of the motor is difficult.
It is desirable that the PWM signal generator is able to control the motor accurately even if the PWM data is generated by the DSP of fast operating speed.
Tagami discloses broadening the pulse width when the pulse width is narrower than a predetermined pulse width. The pulse width is broadened to have the predetermined width in Tagami; therefore, narrow width pulses are not ignored. The minimum width pulses or the narrow width pulses are broadened to have at least the predetermined width. A transformation of pulse width makes a linear speed control based on the PWM signal difficult.